Glass fibers are produced by drawing multiple streams of molten glass at a given rate of speed through orifices or nozzles located in a heated container, known in the fiber glass industry as a bushing. The bushing contains molten glass which is electrically heated and maintained at a given temperature to provide molten glass at a desired viscosity at each of the orifices. The maintenance of a uniform temperature across the face of the bushing; that is, across the area of the bushing on which the orifices are located, is important in providing uniform fiber formation at each orifice.
The fibers drawn from the orifices of a bushing are gathered, after they solidify, into one or more strands, which are then collected on a collet to produce one or more forming packages. In recent years, bushings have increased in size so that bushing having 800 to 2,000 or more orifices are commonplace. It is also common practice to produce more than one strand from larger bushings, and to wind those strands on a single collet to produce corresponding forming packages. Typically, this is accomplished by using one side of a bushing to produce one strand, and the other side to produce a similar second strand. Splitting the bushing in this manner to produce more than one strand requires precise control of the bushing temperature from side to side, so that the strands so produced and collected on a collet will have the same yardage, that is, the same number of yards per pound of glass in a formed package, collected over a given period of time. Even small variations in temperature between the two sides of a split bushing will produce substantial differences in the fibers produced at the orifices, and this will be evidenced by a substantial variation in the weights of the forming packages produced by the respective strands.
In U.S. Pat. Nos. 4,024,336, 4,546,485 and 4,594,087 several control sytems are described which have been developed to address the above problem of temperature control in glass fiber forming bushings. In some instances the devices are costly (U.S. Pat. No. 4,024,336); in other instances they require periodic mechanincal adjustment (U.S. Pat. No. 4,546,485) and in the last mentioned patent, U.S. Pat. No. 4,594,087, which is an improvement over the first two patents referred to herein, the control current is limited by the impedance of the shunt path used. The present invention improves on the operation of the control circuit described in U.S. Pat. No. 4,594,087 by providing increased control current or stated another way, a lower effective shunt path impedance, which in time, improves the performance in split bushing by providing enhanced COV's (coefficient of variations) of package weights obtained from such bushings.